The present invention relates to an image processing device for use in a digital copier, facsimile machine, scanner, image filing apparatus or similar apparatus and, more particularly, to an image processing device capable of preventing unauthorized persons from copying inhibited documents.
Recent progress in image processing and image forming technologies has made it possible to produce even copies of bills as finely as true bills with a digital color copier. Today, a digital color copier capable of identifying special documents prohibited from being reproduced, e.g., bills and securities and preventing them from being copied illegally is available.
To identify special documents of the kind described, image data input to the copier may be compared with a particular registered mark (pattern data) by a pattern matching scheme. If the image data includes the particular mark, the document is determined to be a special document. Alternatively, the identification may rely on the comparison of the configuration of a histogram based on a hue distribution, as taught in Japanese Patent Laid-Open Publication No. 4-54681.
In offices, for example, even ordinary documents other than bills, securities and other special documents are often inhibited from being copied for a secrecy purpose. Let this kind of documents be referred to as inhibited documents for simplicity hereinafter. Usually, inhibited documents dealt with in offices are marked with "TOP SECRET", "NO COPYING" or similar indication and distinguished from other documents which can be copied. However, such a mark is not a substantial implementation for preventing inhibited copies from being copied. Specifically, the precondition for the inhibition is that a person, noticed the mark, sees that the document is an inhibited document and surely observes the inhibition. It is likely that a person copies an inhibited document by a copier, sends it by a facsimile machine, or inputs it in an image filing apparatus.
The conventional method for identifying bills, securities and other special documents may also be used to implement an apparatus for distinguishing inhibited documents from other documents. For example, the apparatus may be constructed to identify the label "TOP SECRET" or "NO COPYING" on a document and inhibit the document from being copied. However, even such an apparatus will fail to identify the inhibited document if a person covers the label with, for example, a paper.
The identification using the configuration of a histogram based on a hue distribution is not applicable to an apparatus handling images in the form of bilevel data, i.e., black and white data. Moreover, the problem with this kind of scheme is that the identification of inhibited documents itself is not satisfactory since numerous kinds of inhibited documents are dealt with in office transactions and since their histograms do not always have a common configuration.